This invention relates to the transmission of digital data in analog form at high speed with a low rate of error due to transmission impairments, including electrical noise.
More particularly the invention provides new signal structures characterized by relatively high immunity to error due to multiplicative noise, as well as additive noise and phase jitter.
Digital data conventionally is transmitted, as on telephone lines, by sending a carrier signal with analog modulation coded to convey, in every time interval, a set of binary digits. A conventional practice sends M bits once per transmission interval, i.e., baud interval, where M is an integer. The M bits have 2.sup.M possible values. Hence the coded analog signal in this practice identifies one of the 2.sup.M possible values once per baud interval. For example, to send 2,400 bits per second (bps) in each time interval, a single bit is transmitted by coding the analog signal to send one of two values, i.e., M=1 and 2.sup.M =2. Examples of conventional higher speed transmission rates are 9,600 bps wherein four bits are transmitted in each interval by sending one of sixteen analog values; and 14,400 bps, wherein six bits are transmitted in each interval by sending one of sixty-four analog values. Conventional baud rates include 1200; 1600; and 2400 symbols per second.
One conventional coded analog signal used for this data transmission uses double sideband-quadrature carrier modulation (DSB-QC). This modulation technique involves two carrier signals 90.degree. apart in phase, i.e., a quadrature relation, and each having double sideband modulation.
The coding of the modulation, in particular the selective amplitude and phase of the modulation, is defined by two analog parameters designated a.sub.i and b.sub.i where (i) is an integer. The values of these modulation parameters identify, in each interval, the one analog value being transmitted. Thus, in a 9,600 bps transmission system, there are sixteen possible sets of the two analog parameters to identify one of the sixteen possible analog values being sent. Similarly in a 14,400 bps transmission system, there are sixty-four possible sets of the two analog parameters.
The values of the modulation parameters are conventionally plotted as points in a two-coordinate complex plane. The set of points for a transmission system is termed a "signal structure", or "signal space," or "signal constellation".
The occurrence of electrical noise and other transmission impairments makes it difficult, however, to determine the value of the modulation parameters at the receiver end of a data transmission system. The probability of error due to this difficulty in value determination increases with the speed of data transmission. This is because higher transmission speeds involve a greater number of possible values, and it is increasingly difficult to distinguish between them in the presence of electrical impairments.
It is known to select a signal structure to reduce error rates and otherwise to enhance data transmission. U.S. Pat. Nos. 3,887,768 and 4,271,527 disclose two designs for signal structures. These and other known signal structures are nevertheless subject to significant error rates in the presence of certain forms of transmission impairments.
It accordingly is an object of this invention to provide a data transmission method and apparatus for communicating data with an improved low error rate in the presence of different forms of transmission impairments.
Another object of the invention is to provide a signal structure for high speed data transmission with relatively high immunity to error in the presence of different forms of transmission impairments.
A further object of the invention is to provide such a method and apparatus, and such a signal structure, which is relatively immune to error due to impairments other than phase jitter and additive noise.
It is also an object of the invention to provide a method and apparatus of the above character, and a signal structure of the above character, which can readily be practiced with competitively priced equipment.
Other objects of the invention will in part be obvious and will in part appear hereinafter.